As is known in the related art slide fastener, an upper blade or a lower blade of a slider has flexibility and flanges which extend from the upper and lower blade is designed. When a lateral pulling force is applied to a pair of fastener stringers, the width of a tape insertion path of a slider is expanded and one of the fastener stringers is separated from the slider (hereinafter, referred to as ‘snapped,’ see e.g. Patent Document 1). Patent Document 1 discloses a slider in which both flanges at edges of one of the upper blade and the lower blade have an inclined shape with a shoulder mouth side thereof being lower than a central portion thereof, and the gap of the tape insertion path gradually narrows from the shoulder mouth side toward a rear mouth side. In addition, Patent Document 1 discloses another slider in which flexibility is increased by forming a recess in the upper surface of the upper blade at one side, and the flanges on one side edge are formed such that the gap of the tape insertion path at the one side edge is wider than the gap of the tape insertion path at the other side edge and is uniform.